The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty.
Silicon Carbide semiconductor material is known to have properties that are of significant advantage over the Germanium, Silicon and Gallium Arsenide materials of present majority use in electronic devices. The large bandgap, the breakdown electrical field characteristics, the upper limit of electron velocity, the high possible operating temperature and the greater thermal conductivity of Silicon Carbide with respect to currently used materials are each areas of significant advantage obtained with use of Silicon Carbide in semiconductor devices.
Ohmic contact formation on Silicon Carbide (SiC) semiconductor material is however a significant concern in the successful fabrication of Silicon Carbide electronic devices. High frequency and high power Silicon Carbide semiconductor devices are especially known to require the use of quality ohmic contacts and are presently limited in performance by the lack of a satisfactory contact arrangement. For example, undesirably large ohmic contact resistance limits the high frequency performance of the Silicon Carbide devices-primarily by introducing undesired contact resistance and reduced current flow in connections to a Silicon Carbide device. It has been determined that contact resistivity must be reduced to about 10xe2x88x926 xcexa9cm2 in order to achieve high frequency Silicon Carbide semiconductor devices usable in 8 to 12 gigahertz X-band radar systems [1,2]. Parenthetic numbers such as these identify references listed in APPENDIX 1 at the close of this specification. Generally, good ohmic contact formation can be presently achieved only with use of extremely highly doped Silicon Carbide semiconductor layers, layers having doping levels in the 1018 or 1019 atoms per cubic centimeter range, and with thermal annealing at around 1000xc2x0 Celsius. Such high temperature annealing however introduces difficulties in device fabrications [3-5]. Although many studies have been published, the mechanism of ohmic contact formation on Silicon Carbide yet remains somewhat unclear.
With respect to the elements used in doping Silicon Carbide semiconductor and without wishing to be bound or limited by questions regarding this somewhat wafer manufacturer-proprietary topic, applicants understand the element nitrogen to be an important dopant used in achieving n-type Silicon Carbide. For present purposes therefore it appears sufficient to state that n-type Silicon Carbide is available in epitaxial wafer form from suppliers to the semiconductor field and that at least one supplier of such material is identified in the paragraphs following herein.
The present invention provides for low electrical resistance ohmic contact with n-type Silicon Carbide semiconductor material.
It is therefore an object of the present invention to provide low electrical resistance n-type Silicon Carbide ohmic contacts that can be achieved at a lower processing temperature than currently used contact arrangements.
It is another object of the invention to provide a usable n-type Silicon Carbide ohmic contact that is achievable with contact related temperatures below 900 degrees Celsius.
It is another object of the invention to provide a usable n-type Silicon Carbide ohmic contact that is achievable with annealing temperatures as low as 700 degrees Centigrade.
It is another object of the invention to provide a n-type Silicon Carbide ohmic contact enabling use of lower Silicon Carbide semiconductor material doping levels than other contact arrangements.
It is another object of the invention to provide a n-type Silicon Carbide ohmic contact in which graphitic sp2 Carbon materials are used to an advantage rather than imposing the usually accepted detrimental effects.
It is another object of the invention to provide a n-type Silicon Carbide ohmic contact employing a specific form of Carbon in its fabrication process.
It is another object of the invention to provide an improved n-type Silicon Carbide ohmic contact that is achieved through use of a metallic catalytic agent during contact fabrication.
It is another object of the invention to provide an improved n-type Silicon Carbide ohmic contact that is achieved through use of a nickel catalytic agent or another metal catalytic agent during contact fabrication.
It is another object of the invention to provide an improved n-type Silicon Carbide ohmic contact achievable with annealing temperatures some three hundred degrees Celsius below those used in present Silicon Carbide device ohmic contact fabrications.
It is another object of the invention to provide an improved n-type Silicon Carbide ohmic contact achievable with semiconductor doping concentrations some two orders of magnitude below those used in present Silicon Carbide device ohmic contact fabrications.
It is another object of the invention to provide an improved n-type Silicon Carbide ohmic contact through the use of nano-sized graphitic structures in the contact region.
It is another object of the invention to provide an improved n-type Silicon Carbide ohmic contact achievable through the predictable decomposition of an initial form of Carbon into a contact-usable different form of Carbon.
It is another object of the invention to provide an improved n-type Silicon Carbide ohmic contact achievable through use of either 4H or 6H Silicon Carbide starting materials.
These and other objects of the invention will become apparent as the description of the representative embodiments proceeds.
These and other objects of the invention are achieved by the method of fabricating a graphitic sp2 Carbon-inclusive ohmic contact for a n-type Silicon Carbide semiconductor device, said method comprising the steps of:
providing a clean surfaced wafer sample of n-type Silicon Carbide of selected doping concentration between 1xc3x971016 and 1xc3x971020 atoms per cubic centimeter;
covering said sample clean surface with a layer of amorphous sp2 and sp3 Carbon mixture;
supplying a layer of Carbon conversion-accelerating catalytic metal over said sample layer of amorphous sp2 and sp3 Carbon mixture;
converting a substantial portion of said catalytic metal covered amorphous sp2 and sp3 Carbon mixture on said sample to metal catalyzed nano-sized graphitic flakes of sp2 Carbon;
said converting step including a heat treating annealing of said sample at a temperature of at least 700 degrees Celsius, a temperature selected in response to said selected doping concentration between 1xc3x971016 and 1xc3x971020 atoms per cubic centimeter;
disposing an external circuit electrically conductive element in contact with a selected portion of said converted, metal catalyzed nano-sized graphitic flakes of sp2 Carbon.